In recent years, polymer resin. materials tend to be applied growingly to general optical components such as lenses for glasses or transparent plates, and optical components for optelectronics, in particular, such as optical components of laser-related devices like optical disk devices for recording sound, image or characters.
This is because optical materials made of polymer resins (called “polymer optical materials” sometimes hereafter) are generally lighter, lower-priced and better in workability and mass-productivity than other optical materials (such as optical glass). In particular, polymer resin materials have a significant merit that molding techniques such as injection molding or extrusion molding are applied thereto easily.
However, usually used conventional polymer optical materials show not a little birefringence when they are used in products produced by applying such molding techniques. This fact, including cause of it, is known broadly.
This is briefly illustrated in FIG. 1. In general, a polymer optical material after undergoing a molding process consists of a great number of units (monomers) 1 forming bonding chains of polymer, units 1 being bonded three-dimensionally in an orientation direction, as shown in the illustration. Then each unit (designated by numeral 1) in almost all every polymer material usually employed as optical material has an optical anisotropy regarding refractive index. That is, refractive index npr for polarized wave component parallel to the orientation direction is different from refractive index nvt for polarized wave component perpendicular to the orientation direction.
Such an optical anisotropy can be expressed by index ellipsoid illustration as known well. According to this way of expression, index ellipsoid mark 2 is shown for each bonding unit 1 in FIG. 1. For instance, in case of polymethyl methacrylate (PMMA), each unit (methyl methacrylate) 1 has a relatively small refractive index for the orientation direction and relatively large refractive index for a direction perpendicular thereto. Therefore, macroscopically saying, index ellipsoid 3 is shaped vertically-long as illustrated. In other words, polymethyl methacrylate has a relation npr<nvt. The difference between them Δn=npr−nvt is called “orientation birefringence value”. Orientation birefringence values Δn of actual polymer materials vary depending on degree of orientation of bonding chains (principal chains) of polymer materials. Provided that the bonding chains (principal chains) are fully extended to an ideally orientated state, orientation birefringence value Δn gives a value called “characteristic orientation birefringence value”. Characteristic orientation birefringence values of typical optical rein materials are shown in Table 1. Since characteristic orientation birefringence value is nothing other than value of Δn under an ideal orientation condition, there is a relation 0<|Δn(real)|<characteristic orientation birefringence value, where Δn(real) is Δn of a actual polymer material.
For instance, polymethyl methacrylate shown in FIG. 1 has a characteristic orientation birefringence value equal to −0.0043 and orientation birefringence value Δn(real) in the actual polymer has as relation −0.0043<Δn(real)<0. It is noted that Δn(real)=−0.0043 (value under an ideal orientation state) and also Δn(real)=0 (value under a completely non-orientated sate) are hardly realized. In a similar way, polystyrene has a relation −0.100<Δn(real)<0. Polyethylene showing a positive Δn has a relation 0<Δn(real)<+0.044.
Hereafter, birefringence depending on orientation of polymer is called orientation birefringence and a direction of long axis of index ellipsoid is called orientation birefringence direction. In addition to this, “sign of birefringence is positive” means that “sign of orientation birefringence value Δn(real)” or “sign of characteristic birefringence value” the same as the former is positive (Δn>0), and in the same manner, negativeness (Δn<0) corresponds to an expression such that “sign of birefringence value is negative”.
TABLE 1CHARACTERISTICORIENTATIONBIREFRINGENCE VALUE;SUBSTANCEΔn = npr − nvtPOLYSTYRENE−0.100POLYPHENYLENE OXIDE+0.210POLYCARBONATE+0.106POLYVINYL CHLORIDE+0.027POLYMETHYLMETHACRYLATE−0.0043POLYETHYLENE TEREPHTHALATE+0.105POLYETHYLENE+0.044
Such expression manner of birefringence can be applied to inorganic fine particles (crystal particles) having shapes such as needle-like shape or ellipse-like shape. If applied, npr is defined as refractive index for polarized wave component parallel to the long axis of the fine particle and nvt is defined as refractive index for polarized wave component perpendicular to the long axis. If Δn=npr−nvt has a positive value, expression such that “sign of birefringence is positive” and if it has a negative value, expression such that “sign of birefringence is negative”.
It should be noted that three axes (a-axis, b-axis and c-axis; c-axis being a long axis) corresponding to crystal structure are defined and na, nb, nc are defined as refractive indexes for polarized wave components parallel with a-axis, b-axis and c-axis, respectively, since refractive index for polarized wave component perpendicular to the long axis is generally not constant. Moreover npr and nvt are defined as npr=nc and nvt=(na+nb)/2. Concrete examples are referred to later.
Orientation-birefringence as described above are not specifically subject to problems in many cases using such optical elements as used in applications in which polarization light characteristics are not important.
To the contrary, for example, in the cases of magneto-optical disks of write/erase type developed recently, since polarized light beams are adopted as reading beam or writing beam, a birefringent optical element (such as disk itself or lens) disposed in a light path affects accuracy of reading or writing.
In usual cases other than the above example, unintentionally existing birefringence is not desirable for many optical elements. Under such background, some attempts for reducing or eliminating birefringence of optical elements. Major examples of them are as follows.
(1) Method disclosed in U.S. Pat. No. 4,373,065; This aims to obtain a non-birefringent optical resin material by blending two kinds of polymer resins which are completely soluble to each other and have signes of orientation birefringence opposite to each other.
(2) Method disclosed in Japanese Patent Laid-Open Tokkai-sho 61-19656; This utilizes an aromatic polycarbonate-type resin composite obtained by mixing aromatic polycarbonate and a specific co-polymers of styrene-type at a specific ratio.
(3) Method disclosed in Japanese Patent Laid-Open Tokkai-sho 62-240901; According to this method, non-birefringent optical resin materials are obtained from mixture of a polymer mainly composed of vinyl monomer units and a polyphenilene ether, or block co-polymer consisting of the polymer parts of both, or mixture of them.
(4) Method disclosed in Japanese Patent Laid-Open Tokkai-sho 61-108617; According to this method, two or more kinds of monomers which have positive and negative main polar coefficients not less than 50×10−25 d undergo random co-polymerization, graft co-polymerization or block co-polymerization.
(5) Method disclosed in “Optics”, Vol.20 Number 2pp. 80(30)-81(31), Febuary 1991; This was proposed by the instant inventor, obtaining a non-birefringent optical resin material by co-polymerizing a monomer mixture of methyl methacrylate (MMA) and trifluoroethyl methyl methacrylate (3FMA), or monomer mixture of methyl methacrylate (MMA) and methyl methacrylate (BzMA). In short, this causes monomers, which give basis for polymers opposite signs of orientation birefringence to be mixed and co-polymerized.
(6) Method disclosed in WO 01/25364; According to this method, which was also proposed by the instant inventor, numerous inorganic fine particles are dispersed in a transparent polymer is subject to an external molding force caused by drawing, with the result that bonding chains of the polymer and the numerous inorganic fine particles are orientated in approximately parallel with each other, thereby cancelling orientation birefringences of the polymer resin and inorganic fine particles. Combination of polymer resin and inorganic fine particles is chosen so that orientation birefringences of them are cancelled when bonding chains of polymer resin and inorganic fine particles (i.e. long axes of them) are orientated in parallel to each other.
However, the above (1) method blending two kinds of polymer resins requires that the polymer resins to be blended are put in molten state or solution state in order to mix them at a high uniformity. Moreover, even if such means are applied, it is very difficult to obtain a polymer resin that actually shows a low birefringence overall without ununiformity.
Still moreover, polymer resin blend composites obtained by this method can not avoid light scattering from being generated due to a natural refractive index difference of the blended polymer resins, making impossible to obtain highly transparent optical materials.
Next, in the above (2) and succeeding methods, those producing low orientation birefringence polymer resins by random copolymerization are expected in principle to provide highly transparent optical materials. However, this method causes monomers giving basis of two or more kinds of polymer reins to be mixed and copolymerized, and accordingly monomer reaction ratio of the monomers must be approximated to 1, being subject to a problem that material combinations satisfying such a condition are extremely rare.
The proposition of the above (5) includes such material combinations, one of which employs a monomer mixture of methyl methacrylate (MMA) and trifluoroethyl methyl methacrylate (3FMA) and involves a drawback that the latter material (3FMA) is extremely expensive.
Further to this, in both methods, one employing copolymerization of methyl methacrylate (MMA) and trifluoroethyl methyl methacrylate (3FMA) and the other employing copolymerization of methyl methacrylate (A) and benzil methyl methacrylate (BzMA), mixture ratio of trifluoroethyl methyl methacrylate (3FMA) or benzil methyl methacrylate (BzMA) to methyl methacrylate (MMA) must be remarkably large for preventing orientation birefringence from emerging.
Mixture ration necessary for cancelling orientation birefringence is MMA/3FMA=50/50 (wt %/wt %) in the former case and MMA/BzMA=80/20 (wt %/wt %) in the latter case. This makes impossible to give the produced materials characteristics equivalent to those of PMMA and the produced materials have mechanical characteristics and transparency inferior to those of PMMA.
Still further to this, in the case of methods utilizing graft co-polymerization included in the above (4), it is difficult to forecast-and-control quantitatively strength of resultant orientation birefringence of the synthesized resins for an employed monomer combination and it can not be judged whether products having well-cancelled orientation, birefringence are obtained or not before causing actual reactions. This makes difficult to provide industrially products having stable characteristics.
Lastly, although the above (6) method is excellent method overcoming many of the problems to which the above (1) to (5) methods subject, a combination of resin material and inorganic fine particles having signs of birefringence opposite to each other must be employed.
Moreover, according to subsequent researches by the instant inventor, it has been found that a resin material and inorganic fine particles are not orientated in some cases to the same direction under flowing conditions such as actual injection molding processes, as described later.
However, non-birefringent materials that can be produced through an injection molding process in such cases have not been developed.